Voltage-gated sodium channels, transmembrane proteins that initiate action potentials in nerve, muscle and other electrically excitable cells, are a necessary component of normal sensation, emotions, thoughts and movements (Catterall, W. A., Nature (2001), Vol. 409, pp. 988-990). These channels consist of a highly processed alpha subunit that is associated with auxiliary beta subunits. The pore-forming alpha subunit is sufficient for channel function, but the kinetics and voltage dependence of channel gating are in part modified by the beta subunits (Goldin et al., Neuron (2000), Vol. 28, pp. 365-368). Electrophysiological recording, biochemical purification, and molecular cloning have identified ten different sodium channel alpha subunits and four beta subunits (Yu, F. H., et al., Sci. STKE (2004), 253; and Yu, F. H., et al., Neurosci. (2003), 20:7577-85).
The hallmarks of sodium channels include rapid activation and inactivation when the voltage across the plasma membrane of an excitable cell is depolarized (voltage-dependent gating), and efficient and selective conduction of sodium ions through conducting pores intrinsic to the structure of the protein (Sato, C., et al., Nature (2001), 409:1047-1051). At negative or hyperpolarized membrane potentials, sodium channels are closed. Following membrane depolarization, sodium channels open rapidly and then inactivate. Channels only conduct currents in the open state and, once inactivated, have to return to the resting state, favoured by membrane hyperpolarization, before they can reopen. Different sodium channel subtypes vary in the voltage range over which they activate and inactivate as well as their activation and inactivation kinetics.
The sodium channel family of proteins has been extensively studied and shown to be involved in a number of vital body functions. Research in this area has identified variants of the alpha subunits that result in major changes in channel function and activities, which can ultimately lead to major pathophysiological conditions. The members of this family of proteins are denoted Nav1.x, where x=1 to 9. Nav1.1 and Nav1.2 are highly expressed in the brain (Raymond, C. K., et al., J. Biol. Chem. (2004), 279(44):46234-41) and are vital to normal brain function. Some loss of function mutations in Nav1.1 in humans result in epilepsy, apparently because many of these channels are expressed in inhibitory neurons (Yu, F. H., et al., Nat Neurosci (2006), 9 (9), 1142-9). Thus, block of Nav1.1 in the CNS may be counter-productive because it can produce hyperexcitability. However, Nav1.1 is also expressed in the peripheral nervous system and block may afford analgesic activity.
Nav1.3 is expressed primarily in the fetal central nervous system. It is expressed at very low levels or not at all in the peripheral nervous system, but expression is upregulated in the dorsal horn sensory neurons of rats after nervous system injury (Hains, B. D., et al., J. Neurosci. (2003), 23(26):8881-92). Thus, it is an inducible target for treatment of pain following nerve injury.
Nav1.4 is expressed primarily in skeletal muscle (Raymond, C. K., et al., op. cit.). Mutations in this gene have been shown to have profound effects on muscle function including paralysis, (Tamaoka A., Intern. Med. (2003), (9):769-70).
Nav1.5, is expressed mainly in cardiac myocytes (Raymond, C. K., et al., op. cit.), including atria, ventricles, the sino-atrial node, atrio-ventricular node and cardiac Purkinje fibers. The rapid upstroke of the cardiac action potential and the rapid impulse conduction through cardiac tissue is due to the opening of Nav1.5. Abnormalities in the function of Nav1.5 can result in the genesis of a variety of cardiac arrhythmias. Mutations in human Nav1.5 result in multiple arrhythmic syndromes, including, for example, long QT3 (LQT3), Brugada syndrome (BS), an inherited cardiac conduction defect, sudden unexpected nocturnal death syndrome (SUNDS) and sudden infant death syndrome (SIDS) (Liu, H., et al., Am. J. Pharmacogenomics (2003), 3(3):173-9). Sodium channel blocker therapy has been used extensively in treating cardiac arrhythmias.
Nav1.6 is a widely distributed voltage-gated sodium channel found throughout the central and peripheral nervous systems. It is expressed at high density in the nodes of Ranvier of myelinated neurons (Caldwell, J. H., et al., Proc. Natl. Acad. Sci. USA (2000), 97(10): 5616-20).
Nav1.7 is a tetrodotoxin-sensitive voltage-gated sodium channel encoded by the gene SCN9A. Human Nav1.7 was first cloned from neuroendocrine cells (Klugbauer, N., et al., 1995 EMBO J., 14 (6): 1084-90) and rat Nav1.7 was cloned from a pheochromocytoma PC12 cell line (Toledo-Aral, J. J., et al., Proc. Natl. Acad. Sci. USA (1997), 94:1527-1532) and from rat dorsal root ganglia (Sangameswaran, L., et al., (1997), J. Biol. Chem., 272 (23): 14805-9). Nav1.7 is expressed primarily in the peripheral nervous system, especially nocieptors and olfactory neurons and sympathetic neurons. The inhibition, or blocking, of Nav1.7 has been shown to result in analgesic activity. Knockout of Nav1.7 expression in a subset of sensory neurons that are predominantly nociceptive results in resistance to inflammatory pain (Nassar, et al., op. cit.). Likewise, loss of function mutations in humans results in congenital indifference to pain (CIP), in which the individuals are resistant to both inflammatory and neuropathic pain (Cox, J. J., et al., Nature (2006); 444:894-898; Goldberg, Y. P., et al., Clin. Genet. (2007); 71:311-319). Conversely, gain of function mutations in Nav1.7 have been established in two human heritable pain conditions, primary erythromelalgia and familial rectal pain, (Yang, Y., et al., J. Med. Genet. (2004), 41(3):171-4). In addition, a single nucleotide polymorphism (R1150W) that has very subtle effects on the time- and voltage-dependence of channel gating has large effects on pain perception (Estacion, M., et al., 2009. Ann Neurol 66: 862-6; Reimann, F., et al., Proc Natl Acad Sci USA (2010), 107: 5148-53). About 10% of the patients with a variety of pain conditions have the allele conferring greater sensitivity to pain and thus might be more likely to respond to block of Nav1.7. Because Nav1.7 is expressed in both sensory and sympathetic neurons, one might expect that enhanced pain perception would be accompanied by cardiovascular abnormalities such as hypertension, but no correlation has been reported. Thus, both the CIP mutations and SNP analysis suggest that human pain responses are more sensitive to changes in Nav1.7 currents than are perturbations of autonomic function.
Nav1.8 is expressed primarily in sensory ganglia of the peripheral nervous system, such as the dorsal root ganglia (Raymond, C. K., et al., op. cit.). There are no identified human mutations for Nav1.8 that produce altered pain responses. Nav1.8 differs from most neuronal Nav's in that it is insensitive to block by tetrodotoxin. Thus, one can isolate the current carried by this channel with tetrodotoxin. These studies have shown that a substantial portion of total sodium current is Nav1.8 in some dorsal root ganglion neurons (Blair, N. T., et al., J Neurosci (2002), 22: 10277-90). Knock-down of Nav1.8 in rats has been achieved by using antisense DNA or small interfering RNAs and virtually complete reversal of neuropathic pain was achieved in the spinal nerve ligation and chronic constriction injury models (Dong, X. W., et al., Neuroscience (2007), 146: 812-21; Lai J., et al. Pain (2002), 95: 143-52). Thus, Nav1.8 is considered a promising target for analgesic agents based upon the limited tissue distribution of this Nav isoform and the analgesic activity produced by knock-down of channel expression.
Nav1.9 is also a tetrodotoxin insensitive, sodium channel expressed primarily in dorsal root ganglia neurons (Dib-Hajj, S. D., et al. (see Dib-Hajj, S. D., et al., Proc. Natl. Acad. Sci. USA (1998), 95(15):8963-8). It is also expressed in enteric neurons, especially the myenteric plexus (Rugiero, F., et al., J Neurosci (2003), 23: 2715-25). The limited tissue distribution of this Nav isoform suggests that it may be a useful target for analgesic agents (Lai, J., et al., op. cit.; Wood, J. N., et al., op. cit.; Chung, J. M., et al., op. cit.). Knock-out of Nav1.9 results in resistance to some forms of inflammatory pain (Amaya, F., et al., J Neurosci (2006), 26: 12852-60; Priest, B. T., et al., Proc Natl Acad Sci USA (2005), 102: 9382-7).
This closely related family of proteins has long been recognized as targets for therapeutic intervention. Sodium channels are targeted by a diverse array of pharmacological agents. These include neurotoxins, antiarrhythmics, anticonvulsants and local anesthetics (England, S., et al., Future Med Chem (2010), 2: 775-90; Termin, A., et al., Annual Reports in Medicinal Chemistry (2008), 43: 43-60). All of the current pharmacological agents that act on sodium channels have receptor sites on the alpha subunits. At least six distinct receptor sites for neurotoxins and one receptor site for local anesthetics and related drugs have been identified (Cestèle, S., et al., Biochimie (2000), Vol. 82, pp. 883-892).
The small molecule sodium channel blockers or the local anesthetics and related antiepileptic and antiarrhythmic drugs interact with overlapping receptor sites located in the inner cavity of the pore of the sodium channel (Catterall, W. A., Neuron (2000), 26:13-25). Amino acid residues in the S6 segments from at least three of the four domains contribute to this complex drug receptor site, with the IVS6 segment playing the dominant role. These regions are highly conserved and as such most sodium channel blockers known to date interact with similar potency with all channel subtypes. Nevertheless, it has been possible to produce sodium channel blockers with therapeutic selectivity and a sufficient therapeutic window for the treatment of epilepsy (e.g., lamotrignine, phenyloin and carbamazepine) and certain cardiac arrhythmias (e.g., lignocaine, tocainide and mexiletine). However, the potency and therapeutic index of these blockers is not optimal and have limited the usefulness of these compounds in a variety of therapeutic areas where a sodium channel blocker would be ideally suited.
Sodium channel blockers have been shown to be useful in the treatment of pain, including acute, chronic, inflammatory and/or neuropathic pain (see, e.g., Wood, J. N., et al., J. Neurobiol. (2004), 61(1), 55-71. Preclinical evidence demonstrates that sodium channel blockers can suppress neuronal firing in peripheral and central sensory neurons, and it is via this mechanism that they are considered to be useful for relieving pain. In some instances, abnormal or ectopic firing can original from injured or otherwise sensitized neurons. For example, it has been shown that sodium channels can accumulate in peripheral nerves at sites of axonal injury and may function as generators of ectopic firing (Devor et al., J. Neurosci. (1993), 132: 1976). Changes in sodium channel expression and excitability have also been shown in animal models of inflammatory pain where treatment with proinflammatory materials (CFA, Carrageenan) promoted pain-related behaviors and correlated with increased expression of sodium channel subunits (Gould et al., Brain Res., (1999), 824(2): 296-99; Black et al., Pain (2004), 108(3): 237-47). Alterations in either the level of expression or distribution of sodium channels, therefore, may have a major influence on neuronal excitability and pain-related behaviors.
Controlled infusions of lidocaine, a known sodium channel blocker, indicate that the drug is efficacious against neuropathic pain, but has a narrow therapeutic index. Likewise, the orally available local anesthetic, mexiletine, has dose-limiting side effects (Wallace, M. S., et al., Reg. Anesth. Pain Med. (2000), 25: 459-67). A major focus of drug discovery targeting voltage-gated sodium channels has been on strategies for improving the therapeutic index. One of the leading strategies is to identify selective sodium channel blockers designed to preferentially block Nav1.7, Nav1.8, Nav1.9 and/or Nav1.3. These are the sodium channel isoforms preferentially expressed in sensory neurons and unlikely to be involved in generating any dose-limiting side effects. For example, there is concern that blocking of Nav1.5 would be arrhythmogenic, so that selectivity of a sodium channel blocker against Nav1.5 is viewed as highly desirable. Furthermore, nearly 700 mutations of the SCN1A gene that codes for Nav1.1 have been identified in patients with Severe Myoclonic Epilepsy of Infancy (SMEI), making this the most commonly mutated gene in human epilepsy. Half of these mutations result in protein truncation (Meisler, M. H., et al., The Journal of Physiology (2010), 588: 1841-8). Thus, selectivity of a sodium channel blocker against Nav1.1 is also desirable.
In addition to the strategies of identifying selective sodium channel blockers, there is the continuing strategy of identifying therapeutic agents for the treatment of neuropathic pain. There has been some degree of success in treating neuropathic pain symptoms by using medications originally approved as anticonvulsants, such as gabapentin, and more recently pregabalin. However, pharmacotherapy for neuropathic pain has generally had limited success for a variety of reasons: sedation, especially by drugs first developed as anticonvulsants or anti-depressants, addiction or tachyphylaxis, especially by opiates, or lack of efficacy, especially by NSAIDs and anti-inflammatory agents. Consequently, there is still a considerable need to explore novel treatment modalities for neuropathic pain, which includes, but is not limited to, post-herpetic neuralgia, trigeminal neuralgia, diabetic neuropathy, chronic lower back pain, phantom limb pain, and pain resulting from cancer and chemotherapy, chronic pelvic pain, complex regional pain syndrome and related neuralgias.
There are a limited number of effective sodium channel blockers for the treatment of pain with a minimum of adverse side effects which are currently in the clinic. There is also an unmet medical need to treat neuropathic pain and other sodium channel associated pathological states effectively and without adverse side effects due to the blocking of sodium channels not involved in nociception. The present invention provides methods to meet these critical needs.